This invention relates to the field of electrical keyboards. More particularly, this invention relates to the field of membrane keyboards having a plurality of snap action domes which provide tactile feedback to the keyboard operator.
Membrane keyboards of the general type with which this invention is concerned are well known in the art. These keyboards conventionally have a pair of circuit layers, one fixed and one movable, separated by a spacer or separator layer. The circuit layers are sheets of insulating material, with circuit patterns thereon. These circuit patterns face each other and are separated by a spacer, which has apertures at the location of aligned contact elements on the fixed and movable circuit sheets. Typically, one circuit layer will have a plurality of snap action domes formed therein which provide tactile feel or feedback to the keyboard operator. Electrical switching is effected by applying finger or other pressure to specific locations on one of the circuit sheets to move a contact on that circuit sheet through an aperture to make contact with a contact element on a snap action dome of the other circuit sheet whereby the dome inverts thereafter transmitting a snap action to the operator. The fixed and movable circuit layers and the spacer may be separate sheets of material, or any two or three of those sheets may be formed from a single sheet of material folded over in any desired fashion. Keyboards of this configuration are generally formed in a laminate construction with the layers bonded together, sealed or otherwise fixed against relative lateral movement between the layers. The assembly may also include an overlay sheet with indicia of one kind or another to identify key locations and a backer plate to support the assembly.
One disadvantage of a conventional keyboard having snap action domes as described above lies in the quality of desired tactility transmitted to the operator. It has become apparent that the tensile or actuation force which must be exerted on the two circuit layers and insulating spacer during switch operation is very large. As a result, the input pressure and the snap action restoring force are adversely affected. Tactile feedback is generated by the difference between the initial input pressure and the subsequent restorative force. This difference is defined in terms of the snap ratio. In the prior art, both the snap ratio and tactile feel or feedback are particularly adversely affected by the above described conventional structure because the increase in the snap action restoring force is greater than and overrides the increase of the operational pressure.